1. Field of the Invention
The present invention relates to a torque sensor with a first signal generator, whose output signal changes as a function of a torque, a second signal generator, whose output signal changes in the opposite direction as a function of the torque, and a circuit arrangement which has a difference-forming unit and a summing device. In addition, the invention relates to a method of producing a torque-dependent signal from output signals of two signal generators with opposed signal waveforms.
2. Description of the Related Art
A torque sensor and a method of the type cited at the beginning are disclosed by EP 0 765 795 A2. There, a torque-dependent signal is needed in order to operate power-assisted steering. The torque sensor is arranged on the steering column here and determines when a torque is applied to the steering column. Depending on the torque determined, an additional drive is activated, which moves the steered wheels in such a way that the torque applied to the steering column is minimized. The torque sensor operates with a number of transmission paths which are arranged one after another, are supplied from a transmitter and at whose end the two signal generators are arranged. If a torque is applied in one direction, the output signal from one signal generator increases, and the output signal from the other signal generator decreases. In the case of a torque in the other direction, this sequence is reversed.
In order to prevent changes in the transmission path which have nothing to do with a torque change having a negative influence on the torque signal determined, the sum of the output signals is formed and fed to a controller. The controller then controls the gain of an amplifier which supplies the feed signal for the transmission path, specifically in such a way that the sum of the output signals corresponds to a predefined reference value.
EP 0 555 987 B1 shows a similar arrangement having a torque sensor which, with the aid of a dividing device, divides the difference between the two output signals by their sum, in order to achieve a normalization which, for example, is intended to compensate for aging influences of components.
The present invention provides a method and structure for easily generating a torque signal. In a torque sensor of the type cited at the beginning, this is achieved by the output signal of each signal generator passing through an amplifier whose gain is controlled and is the same for both output signals, and the amplified output signals being present on the difference-forming unit and on the summing device, the output of the summing device being connected to a controller which controls the gain.
In this configuration, the advantage is that the entire control can be kept within one subassembly. It is therefore no longer necessary to go out outside in order to change the gain of a transmitter. Nevertheless, a torque signal is obtained in a simple way and is largely independent of changes which proceed slowly over time, for example aging phenomena of components. Controlling the gain means that influences of this type are compensated for. Furthermore, the further processing of the output signals becomes somewhat simpler, since said signals are necessarily amplified. Accordingly, the xe2x80x9ctransmitted powerxe2x80x9d of a transmitter which effects the output signal in the signal generators can be kept low.
It is preferable for each signal generator to be connected to its own amplifier. This is a very simple possibile implementation. Each output signal can be provided with its own path. The gain can be adjusted by amplifier feedback. Feedback of this type can be changed in a simple way, for example with the aid of a field-effect transistor (FET).
In this case, it is particularly preferred for the gain to be controlled with the aid of a monolithic dual FET. Using the latter, very exact equality of the gains can be achieved, above all if the two amplifiers are also of monolithic construction.
In an alternative configuration, the amplifier is connected to the two signal generators via a changeover switch, the summing device and the difference-forming unit processing consecutive input signals to form sum and difference signals respectively. The only precondition for this is that the time constant of the control of the gain is significantly greater than the changeover period, so that it is possible in each case to process the output signal of the two signal generators with the same gain. This ensures that both output signals are treated identically.
It is preferable if the summing device and the difference-forming unit have a smoothing element, in particular an accurate-phase mean-value rectifier. Since the two output signals, following their amplification, are no longer present simultaneously, but alternately, it must be ensured that they can nonetheless still be processed together. Use is made for this purpose of the smoothing element, in particular a mean-value rectifier operating in phase with the changeover switch. Although the summing device will then no longer receive a signal which corresponds to the sum of the output signals, but a correspondingly smaller signal, for example one half as large, this can be taken into account by appropriate selection of the reference variable. Basically, the sum does not involve absolute values, either. All that is desired to be achieved is that the difference between the output signals is free of undesired interference. When forming the difference, the mean-value rectifier can actually determine a value which corresponds to the difference between the two output signals.
In an alternative configuration, the object is also achieved by the difference-forming unit being connected to the signal input and the summing device being connected to the reference input of an analog/digital converter.
This configuration has the advantage that the torque-dependent measured value is available immediately in digital form, which, for example, makes processing in a microprocessor simpler. The fact that the summing device is connected to the reference input of the A/D converter also means that undesired changes in the overall transmission path are eliminated. The sum signal changes in the same way as the output signals whose difference is considered. If the difference output signals are correspondingly normalized to the sum signal, the desired effect is obtained. In this case, neither a division nor the control of a gain is necessary.
It is also advantageous if the signal generators are each formed by an opto-electronic arrangement having a light source and light receiver, the light source changing its transmitted intensity under cyclic control and provision being made for a switch arrangement each, controlled at the same cycle rate, feeding the output signals of the signal generators from cycles of one intensity, in each case inverted in relation to the output signals from cycles of another intensity, to the summing device and the difference-forming unit. In the case of an opto-electronic arrangement, there may be a series of interfering influences, for example variable dark currents, interfering light or amplifier errors, which can largely be eliminated with the preferred configuration. The output signals of the signal generators each have a xe2x80x9cuseful componentxe2x80x9d and an xe2x80x9cinterference componentxe2x80x9d. The useful component varies with the intensity with which the light source radiates or transmits. The interference component remains the same. Since the transmission behavior of the light source is known, the interference components can be calculated out or filtered out in a simple way. The remaining signal is then the true useful signal.
In this case, it is particularly preferred for the light source not to emit any light in each second cycle. This simplifies the evaluation quite considerably.
In a method of the type cited at the beginning, the object is achieved by the two output signals being amplified with the same gain, and the gain being controlled such that a signal corresponding to the sum of the amplified signals corresponds to a predetermined set point.
As explained above in conjunction with the torque sensor, in the case of this procedure it is possible to remain within one subassembly and nonetheless largely to eliminate the interfering influence of external variables, such that a torque-dependent signal which is largely error-free is produced.